1. Field of the Invention
The invention relates to a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, having an injector housing which communicates with a central high-pressure reservoir and in which a nozzle needle that cooperates with a valve piston which is guided in a valve piece is axially displaceable.
2. Description of the Prior Art
In common rail injection systems, a high-pressure pump pumps the fuel into the central high-pressure reservoir, which is known as a common rail. In the high-pressure reservoir, high-pressure lines lead to the individual injectors, which are assigned to the various engine cylinders. The injectors are triggered individually by the engine electronics, each via a respective control valve. When the control valve opens, fuel subjected to high pressure flows past the nozzle needle, which at that time is raised, into the combustion chamber.
In conventional injectors, of the kind described for instance in European Patent Disclosure EP 0 604 915 B1, the nozzle needle is triggered by a valve piston via a thrust piece. The valve piston is guided in a valve piece, which is secured in the injector housing. The axis of symmetry of the valve piston does not necessarily in practice match that of the nozzle needle. The result can be increased wear at the nozzle needle guide. Furthermore, contact between the thrust piece and the injector housing in operation can cause changes in friction, which can impair the operating performance of the injector, especially at low rail pressures.
The object of the invention is to increase the service life of the known injectors by simple means. Nevertheless, it should be possible to produce the injector of the invention economically.
In a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, having an injector housing which communicates with a central high-pressure reservoir and in which a nozzle needle that cooperates with a valve piston which is guided in a valve piece is axially displaceable, this object is attained in that the end of the nozzle needle toward the valve piston protrudes into a guide sleeve, in which the end, toward the nozzle needle, of the valve piston or of a thrust rod triggered by the valve piston is received.
To increase the service life of the injector, the end toward the nozzle needle of the valve piston or of a thrust rod triggered by the valve piston is guided coaxially to the axis of symmetry of the nozzle needle.
This assures that the closing force is always introduced centrally into the nozzle needle, and an undesired tilting moment on the nozzle needle is averted. The thrust piece used in conventional injectors can be omitted. Hence the undesired change in friction described above can no longer occur.
In a particular embodiment of the invention, the elongated valve piston in conventional injectors is divided up into a short valve piston, which absorbs the hydraulic forces from the control chamber and seals off the control chamber from the low-pressure region, and a thrust rod, which serves to transmit force from the valve piston to the nozzle needle. The pivotable disposition of the thrust rod can be attained for instance by providing that the thrust rod tapers on its end toward the valve piston.
The above-stated object is also attained in that the valve piston, below its guidance in the valve piece, is deflected elastically out of its axis of symmetry, which is predetermined by the axis of symmetry of a guide in the valve piece. If the bending elasticity of the valve piston is already slight enough, then the valve piston can be used unchanged in the injector of the invention. However, if the bending elasticity of the valve piston and thus also the forces required to deflect its end toward the nozzle needle are too high, then below its guidance in the valve piece the valve piston is tapered, which reduces its bending elasticity. The bending elasticity allows a slight deflection of the end, toward the nozzle needle, of the valve piston out of its axis of symmetry toward the axis of symmetry of the nozzle needle. This assures that any axial offset that may exist between the valve piece and the nozzle needle can be compensated for. That in turn assures that the closing force is always introduced centrally into the nozzle needle, and an undesired tilting moment on the nozzle needle is prevented.
The aforementioned object is also attained in that in the end of the nozzle needle toward the valve piston, a blind bore is embodied centrally, in which the end of the thrust rod, or of the valve piston, toward the nozzle needle is received. This assures that the closing force is introduced centrally into the nozzle needle.
The above object is also attained in that on the end of the nozzle needle toward the valve piston, a thrust peg is embodied, which protrudes into a guide sleeve in which the end of the thrust rod or of the valve piston toward the nozzle needle is received. This assures that the closing force is introduced centrally into the nozzle needle.
Another particular embodiment of the invention is characterized in that between the guide sleeve and a nozzle spring, on the face end of the guide sleeve remote from the nozzle needle, there is a bearing disk, which forms an abutment for the nozzle spring. The bearing disk serves to introduce the closing force of the nozzle spring into the nozzle needle. The nozzle spring serves to exert a defined closing force on the nozzle needle even when the system is pressureless.
Another particular embodiment of the invention is characterized in that the guide sleeve, on its face end remote from the nozzle needle, has a collar which forms an abutment for the nozzle spring. The collar serves to introduce the closing force of the nozzle spring into the nozzle needle.
Another special embodiment of the invention is characterized in that the dimensions of the guide sleeve, on its face end remote from the nozzle needle, are adapted to the dimensions of the nozzle spring. As a result, it is attained that the closing force of the nozzle spring is introduced into the nozzle needle without the formation of a collar and without using a bearing disk. The prestressing force of the nozzle spring can be adjusted by way of the thickness of the bearing disk, the thickness of the collar, the length of the guide sleeve, or by way of a further shim between the nozzle spring and its bearing place in the injector housing.
Another particular embodiment of the invention is characterized in that the guide sleeve, on its face end toward the nozzle needle, has a cylindrical recess on the inside. The cylindrical recess serves to receive one end of the nozzle needle.
Another particular embodiment of the invention is characterized in that an adjusting piece is disposed between the nozzle needle and the thrust rod or the valve piston. By the use of graded adjusting pieces, it is possible to adjust the nozzle needle stroke.